x/text: overwriting a previous write

[ondrejbudai]

This line of code of code has no effect because the value in precedingCharacterType is overridden later.

[davecheney]

@ondrejbudai feel free to send a PR if the tests pass after removing this line

[cagedmantis]

/cc @mpvl

[gopherbot]

Change https://golang.org/cl/358834 mentions this issue: text/unicod/bidi: remove duplicate assignment